Cdk throws runtime error "no 'Public' subnet groups in this VPC" while creating an rds cluster.

Question

I created a VPC through aws cdk python library. It contains 3 public and 3 private subnet along with Internet Gateway attached and route table association set up. There is also a NAT gateway set up and attached for one of the private subnets. I used aws_ec2.CfnSubnet for configuring this, since normally while declaring subnet configuration inside VPC doesn't let us specify CIDR block. And also CfnSubnet doesn't have a subnet type parameter which can be set to avoid this issue. Now I am trying to setup an rds cluster in that vpc but cdk throws an error saying "There are no 'Public' subnet groups in this VPC. Available Types:". In fact the available types shown are also empty. I don't know how that can be. So what is the solution for this? Why can't I create the rds cluster within existing subnets?

This might be solved if I am able to:

specify subnet type inside CfnSubnet
specify subnet type later on in the code
setup rds cluster with existing private/public subnet

Public Route Table: Enter image description here

Private Route Table: Enter image description here

This might be solved if I am able to:
* specify subnet type inside [CfnSubnet](https://docs.aws.amazon.com/cdk/api/v2/python/aws_cdk.aws_ec2/CfnSubnet.html) 
* specify subnet type later on in the code
* setup rds cluster with existing private/public subnet

Public Route Table:
![Enter image description here](/media/postImages/original/IMrFtOFPfHQzG75gkJr4q2YQ)

Private Route Table:
![Enter image description here](/media/postImages/original/IMxPxX3En5TCOlPXL_J4N9Tg)

Answer

So there's a development, I think I found a solution. Previously I was setting up the database cluster in the same cdk stack where I was creating VPC, subnets, route tables, etc. I was creating the cluster after these resource creations, but it was throwing the above mentioned error. The error thrown there also didn't list any available subnet types. So I deployed the stack without the cluster creation code. I created a new stack and put the code there and it got deployed smoothly. Thus it seems I can't put the DB Cluster creation code in the same stack where I'm creating the VPC for the same. Can somebody explain why that's the case?

Answer

Did you ensure the public subnets' route table has a route for 0.0.0.0/0 targeting the IGW?  Should be a similar setup for the private subnets with the 0.0.0.0/0 route targeting the NAT GW.

Cdk throws runtime error "no 'Public' subnet groups in this VPC" while creating an rds cluster.

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